1. Field of the Invention
This invention relates to a method and apparatus for concentration and recovery of halocarbons from process effluent gas streams. The present invention further relates to a process of removing impurities from halocarbon-containing waste streams to enhance such concentration and recovery process.
2. Description of the Related Art
Fluorinated and chlorinated compounds are used in semiconductor etch, CVD, and cleaning processes. Examples of compounds which are widely used include perfluorocarbons, fluorinated hydrocarbons, and chlorofluorocarbons as well as sulfur hexafluoride, which will also be referred to herein as a "halocarbon." Examples are C.sub.2 F.sub.6, CF.sub.4, CHF.sub.3, and SF.sub.6. These compounds are suspected of causing global warming by a greenhouse effect. While long-term effects are unknown, current data suggest that compounds of this type are accumulating in increasing concentrations in the upper atmosphere, and that they can persist there for thousands of years. Manufacturers of many of these compounds, especially the perfluorocarbons (PFCs) are establishing policies that require customers to ensure that a high percentage of the PFCs are recycled or prevented from reaching the atmosphere.
Incineration as been shown to be an effective means of destroying halocarbons including PFCs. However, incineration requires considerable capital investment. In addition, because of the stability of PFCs, they must be heated to over 1200.degree. C. before oxidation occurs. The reaction that takes place: EQU C.sub.2 F.sub.6 +2O.sub.2 +3H.sub.2 .fwdarw.2CO.sub.2 +6HF
produces toxic and corrosive hydrogen fluoride, which must be neutralized. Typically, a wet scrubber would be used, adding to the overall cost and complexity, and generating large volumes of aqueous waste whose disposal may be inconvenient. Also, burning processes that involve hydrogen raise safety which the halocarbon is to be recovered causes problems, in particular, it limits the pressure at which the sorption step takes place to about one atmosphere. Such adsorption can be more efficient at higher pressures.
Recovery of fluorocarbons from semiconductor etch, CVD, and cleaning process effluents is, moreover, complicated by the presence of other toxic or corrosive components in the gaseous waste stream. Typically, the other waste gases can include corrosive species such as HF, or in for tungsten etch processes, WOF.sub.3. Other species present can include SiF.sub.4, F.sub.2, or COF.sub.2. Plasma processes may generate a large variety of by-product species because of chemical reactions such as rearrangement or scrambling that can occur in the high energy plasma. The table below presents a list of possible species in the exhaust of a reactor in which C.sub.2 F.sub.6 is used in a plasma process for cleaning steps in silicon processing. Many of these materials are hazardous to both personnel and equipment because of their corrosivity and in some cases toxicity.
__________________________________________________________________________ AMI-5000 CVD CHAMBER CLEAN LIST OF POSSIBLE SPECIES IN EXHAUST STREAM Vapor Pressure Boiling Critical Name Formula MW KPa T .degree.C. T .degree.C. T .degree.C. __________________________________________________________________________ Carbonyl fluoride COF.sub.2 66.007 5,620 21.1 -84.6 22.8 Carbon dioxide CO.sub.2 44.011 -- -- -- 31.1 Carbon monoxide CO 28.010 -- -- -191.5 -140.2 Carbon tetrafluoride CF.sub.4 88.005 1.33 -169 -128.0 -45.6 Decafluorobutane C.sub.4 F.sub.10 238.028 330 31.7 -2.0 113.2 1,1-Difluoroethylene H.sub.2 C.sub.2 F.sub.2 64.035 3,571.5 21.1 -85.7 29.7 Dioxygen difluoride F.sub.2 O.sub.2 70.0 1.33 -120 -- -- Fluorine F.sub.2 37.997 -- -- -188.1 -128.8 Hexafluorodisilane Si.sub.2 F.sub.6 170.162 0.0133 -96 -- -- Hexafluoroethae C.sub.2 F.sub.6 138.012 3,070 21.1 -78.2 19.7 Hexafluoropropylene C.sub.3 F.sub.6 150.023 687.4 21.1 -- 94.0 Hydrogen fluoride* HF 20.006 103 20 19.5 188.0 Octafluorocyclobutane C.sub.4 F.sub.8 200.031 274 21.1 -5.8 115.3 Octafluoropropane C.sub.3 F.sub.8 188.021 -- -- -- -- Oxygen difluoride F.sub.2 O 53.996 0.0133 -205 -144.9 -58.0 Perfluoroisobutylene C.sub.4 F.sub.8 200.031 -- -- -- -- Silicon tetrafluoride SiF.sub.4 104.08 0.0133 -155 -- -14.2 Tetraflouroethylene C.sub.2 F.sub.4 100.016 3,040 21.1 -76.3 33.3 __________________________________________________________________________
The presence of these other contaminants in process effluents can cause serious problems in adsorption-based recovery/recycle systems. The void space of the adsorbent can be filled with reaction products. This fouling of the adsorbent bed can destroy its ability to be regenerated, which seriously degrades the economics of recovery/recycle. In addition, the feed stream to for a recycling process needs to be as clean as possible to make recycle economically attractive. Allowing these corrosive contaminants to remain in the gaseous effluent stream can lead to corrosion of ductwork and valves, and eventually to system failure. And finally, environmental regulations prohibit venting significant quantities of HF or the other corrosive gases to the atmosphere.
In addition to these by-products generated from halocarbon starting materials in the wafer processing step, other hazardous components may be present in the effluent gas stream along with the halocarbon. Corrosive, reactive, flammable and/or poisonous gases may be used in processing steps along with the halocarbon or the effluent from which becomes mixed with the halocarbon-containing effluent gas stream. Examples include arsine (toxic) used in metalorganic chemical vapor deposition, boron trichloride (corrosive) used in etch or cleaning steps, silane (extremely flammable) used in silicide deposition, and tantalum pentaethoxide (solid-forming: rapidly hydrolyzes) used to deposit tantalum oxide.
Providing a relatively clean, dry, and inert incoming stream to a concentration and recovery unit will help to maximize its capacity as well as avoid hazards to personnel and equipment.
Removal of the contaminants from the process effluent stream can be accomplished by water-based scrubbers. Using wet scrubbing, the exhaust gas stream may be processed using aqueous solutions of relatively cheap reagents such as sodium hydroxide or potassium permanganate. However, wet scrubbing requires a large gas treating unit and the resulting large volumes of aqueous waste solutions may present problems as to environmentally acceptable disposal. In addition, the aqueous solutions used are very corrosive, and they thus can corrode fittings and connections and present risks to personnel and equipment in the event of an equipment failure. Water scrubbers require constant maintenance, and if the gaseous effluent contains highly flammable compounds such as silane (which is not soluble in water but reacts readily with oxygen), they present an explosion hazard. Moreover, for an adsorption-based concentration and recovery system to operate economically, the water vapor would then need to be removed from the scrubber effluent, or else the adsorption capacity of the sorbent bed would be largely consumed by water molecules which compete for the same adsorption sites. In addition, in most cases water would not desorb as readily as the halocarbon component from the adsorption bed. Therefore, water vapor contamination will, over time, seriously compromise the effectiveness of the concentration and recovery unit.
Dry scrubbing is preferred as a method for providing a relatively clean gas stream for recovery and recycle of halocarbons, because it does not contribute water vapor to the effluent stream. In addition, dry scrubbers operate passively and so do not require much energy or complicated equipment design.
With dry scrubbing, the flow characteristics and therefore the contact time of the gas with the scavenger may be adjusted by varying the porosity or particle size of the scavenger or selection of appropriate support material. In applications involving high flow rates, the scavenger may thereby be tailored for high kinetic efficiency; conversely under conditions of low flow, a highly loaded, high capacity support may be selected. To gain the advantages of both high efficiency and high capacity, scavenger beds may comprise more than one type of scavenger combination, in layered or mixed form.
A good effluent gas scrubber must not only remove hazardous gas components to low levels, preferably below their TLV/PEL limits, but must also possess several other attributes. It must operate safely, with no risk of explosion or spillage. It should have high capacity for the hazardous components, so that it need not require an extremely large volume for scrubbing or frequent change-outs. It must have high kinetic efficiency for scrubbing, so that high flow rate effluent gas streams may be scrubbed. Scrubbers of a simple, passive design are preferred, since they are likely to be more economical. Finally, the scrubber should convert the hazardous components of the effluent gas stream to stable, environmentally acceptable species that may be disposed of safely and economically.
In order to make recycling economically attractive, the concentration and recovery unit must be able to provide a concentrated stream of halocarbon for further purification or processing. Concentration of the recovered halocarbon gas or vapor may be accomplished by compression or liquefaction.
Accordingly, it is an object of the present invention to provide a method and apparatus for concentration and recovery of volatile halogenated compounds such as perfluorocarbons, fluorinated hydrocarbons, chlorofluorocarbons, and SF.sub.6, all generically referred to herein as halocarbons. The method and apparatus are suitable for use with process effluents that may be contaminated with corrosive by-products, and is able to provide a clean, concentrated stream of recovered halogenated compound, suitable for recycle.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and claims.